Method for making a dental element

ABSTRACT

The present invention relates to a method for fabricating a functional dental element, such as a crown. According to the invention, use is made of a three-dimensional printing technique. The major advantages of the invention are that no mold is needed anymore, which entails a considerable saving of costs, that a great accuracy is achieved, and that the element can be made of different materials.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The invention relates to a method for making a functional dental elementand to a dental element obtainable by such method.

(2) Description of Related Art

Dental elements, such as crowns, are used in clinical practice mainlyfor replacing or correcting dental structures. This can involve partlyor wholly lost teeth or molars. To date, materials for such elementshave been examined in particular for technological/physical and chemicalproperties. Currently, in addition, the biological aspect plays anincreasing role.

Dental elements can be fabricated from different materials. Examplesinclude polymers, metals, composites, combinations of porcelain andmetal, porcelain and other ceramic materials. Glass and ceramicmaterials form an ideal group of materials for dental elements, becausethey are hard, have a high wear resistance, are chemically inert in manymedia (biocompatibility), and can be simply formed into an aestheticdental element. A broad application of these materials, however, isimpeded by the inherent brittleness which is often the result oflimitations in the fabricating process and of the material choice.Recent developments have led to different ceramic systems, such assintered ceramic, glass-infiltrated ceramic and glass-ceramic of variouscompositions, which are less brittle.

The fabrication of dental elements in practice is a complex and timeconsuming affair. The products involved are fabricated on an individualbasis since the exact form of the element is different for every toothor molar in every individual. Conventional techniques that have beenused often utilize a mold. Since this mold can typically be used onlyonce, it will be clear that these techniques are very costly.

In the past, techniques have been proposed which supposedly enablesimplification of the fabricating process of dental elements. Thus, Abeet al., in Int. J. Japan Soc. Prec. Eng., vol. 30, no. 3, 1996, pp.278–279, have proposed to carry out a selective laser sintering (SLS)with titanium. This technique, however, often gives rise to shrinkage.Also, microcracks may be formed, which renders the technique unsuitablefor the fabrication of functional dental elements. In European patentapplication 0 311 214 it has been proposed to make a crown by milling.Milling does not provide the possibility of making colored elements.Moreover, the choice of suitable materials that can be processed bymilling is limited. As noted, ceramic materials form an ideal group ofmaterials for fabricating dental elements, because they are hard, highlywear-resistant and inert under many conditions.

U.S. Pat. No. 5,690,490 describes a method for fabricating a conceptmodel for a dental element by so-called pinhead molding. The methodconcerns the use of a kind of matrix printing technique, wherebymaterial is sprayed on. The printer is controlled with a CAD/CAMprogram. The data which this program utilizes have been obtained from alaser scan of the tooth or the molar to be replaced.

In U.S. Pat. No. 5,823,778, a method is described for the fabrication ofa dental element whereby an impression of the teeth of a patient isobtained, which is subsequently used as a mold to make a copy of adental element. This element is broken down in layers and each layer isscanned to obtain a three-dimensional computer model of the dentalelement.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a technique wherebyfunctional dental elements can be fabricated in a flexible and efficientmanner. Another object is for the technique not to utilize a mold, andto enable making dental elements of polymeric, metallic or ceramicmaterial, or of combinations thereof.

Surprisingly, it has presently been found that the stated objects areachieved by fabricating a dental element utilizing a three-dimensionalprinting technique.

Three-dimensional printing techniques are known per se, and describedinter alia in European patent application 0 431 924, U.S. Pat. No.5,902,441 and international patent applications 94/19112, 97/26302 and98/51747. For a description of the details of the technique, referenceis made to the documents mentioned, which are therefore to be understoodto be inserted herein.

The method according to the invention is in principle suitable forfabricating all types of dental elements. Examples include crowns (frontand lateral teeth), inlays, overlays, onlays, partial crowns, fixationsand veneers.

DETAILED DESCRIPTION OF THE INVENTION

Preferably, in a patient in whom a dental element is to bereplaced/placed, it is first accurately measured what shape the elementis to have. Often, if possible, the starting point will be the shape ofthe tooth or molar, or the portion thereof that is to be replaced. It ispreferred that measurement can take place in a manner which causes thepatient as little inconvenience as possible. Particularly suitabletechniques for measuring the shape for the dental element make use ofoptical scan techniques, in particular the use of lasers. In electronicform, data about the desired shape and dimensions are thereby obtained,which can be directly visualized in a computer. The electronic dataabout the shape and dimensions of the dental element are preferably usedby a computer to control the execution of the three-dimensional printingtechnique. Another suitable method for measuring is by theCEREC-technique, Sirona Dental Systems GmbH, Bensheim, Germany.

In the three-dimensional printing technique, a suitable material isapplied successively in layers, while specific steps are taken to ensurethat each layer adheres to the preceding layer only at particulardesired points. These specific steps are determined by the desired shapeof the dental element and preferably controlled by the above-mentionedelectronic data.

According to the invention, in the specific steps mentioned, use is madeof a binder. This binder is applied to a preceding layer only at thedesired specific points. When to the binder a layer of, for instance,ceramic material from which the dental element is to be shaped, isapplied, this will adhere only to the desired points. The non-adheringpowder, which, accordingly, does not come into contact with the binder,can be simply removed.

The binder is preferably applied to the desired points by means of aprint head, controlled by the computer on the basis of the data obtainedupon measurement. Thereafter, a powder of the material that has beenselected for the fabrication of the dental element is applied.

It is also possible to work upside down and to provide a layer of binderon the bottom side of a plate and subsequently to dip the binder in thepowder. In this last variant, in a simple manner, different kids ofpowder can be used for different layers. In both cases, the powder willbind only at points where binder has been applied. By repeating thesesteps sufficiently often, eventually the desired shape of the dentalelement is obtained. Finally, the binder can be removed by sintering.

According to an alternative to this method, first loose powder is laidin a powder bed, and thereafter binder is applied locally to obtainbinding at the desired points. So, in fact, binder and powder can beapplied in any sequential order.

The substrate on which work is done can be formed by a few layers ofloose powder, so that the dental element to be formed can be readilydetached from the substrate. In sintering, preferably a non-adheringsubstrate, for instance a metal plate, is used.

By virtue of the accuracy of the data that can be obtained by measuringwith the aid of a laser technique, and by virtue of the accuracy withwhich a computer, on the basis of those data, can control a print head,the desired shape and dimensions of the dental element can be obtainedin a highly accurate manner. While in the old-fashioned techniques itwas necessary to additionally shape a dental element several times afterit had been formed in a mold, in the method according to the inventionit normally suffices to carry out additional shaping a single time.Depending on the material that has been selected for the dental element,this additional shaping can be carried out by grinding, filing,polishing, sanding, blatting or by using a ball bed (a vibrating boxcontaining abrasive balls).

The binder that is used in a method according to the invention should besoluble in a suitable solvent to a solution having a viscosity of 1–40mPas, preferably about 3 mPas, and a loading degree of 3–10 wt. %. Thusthe binder preferably has a relatively low molecular weight. Examples ofsuitable binders are colloidal silica, polyvinyl acetate (PVA), starchadhesives, acrylates, polyvinyl alcohol, polyethylene oxide (PEO),ethylenevinyl acetate (EVA) and derivatives thereof.

In the binder, often a colorant will be used. Suitable colorants arenormally based on inorganic pigments having a high content of SiO₂,which renders them heat-resistant. These substances are known per se andcommercially available, for instance, from Carmen, Esprident GmbH,Ispringen, Germany, or VITA Zahnfabrik H. Rauter GmbH & co., BadZäckingen, Germany. Preferably, one or more of the following colorantsare used: SiO, CoO, ZnO, Cr₂O₈, TiO₂, Sb₂O₃, Fe₂O₃ and MnO₂. Dependingon the desired dental color, colorants are preferably used in amounts ofup to 10% by weight, based on the weight of the powder. It is aparticular advantage of the invention that at different points in thedental element, different colors can be used, if desired with atransparent outer layer, yielding a natural optical depth action. Byvirtue of these and other advantages, a dental element resembles a realtooth or molar extremely faithfully.

As noted, this binder can be applied to a suitable substrate with aprint head. The print head is controlled by a computer on the basis ofthe data which have been obtained through prior measurements on thepatient for the purpose of the dental element. Examples of suitableprint heads are, for instance, inkjet heads of the continuous or of thedrop-on-demand type. The print head preferably has a spray nozzle of adiameter between 10 and 100 μm, more preferably between 25 and 75 μm anda length between 50 and 150 μm.

The powder that is used is selected on the basis of the material ofwhich the dental element is eventually to be made. The powder can beused both in dry form and in dispersed form (slurry). Dispersions arepreferably prepared in water or an aqueous solution. In addition, someorganic solvents, such as isopropanol, can be used. The skilled personwill be able to choose a suitable solvent on the basis of his normalknowledge. Depending on the particle size of the powder, it may bedesirable to prepare a colloidal solution of the powder, for instance byaddition of a base, salt and/or surfactant. When the powder is appliedin dispersed form, preferably a drying step takes place each time beforea next layer is applied.

According to a preferred embodiment of the invention, in each layer,several materials, of a different nature, are used. It is also possible,and highly favorable under certain circumstances, to modify thecomposition of the powder per layer to be applied. If per layer one typeof material is applied, often a doctor blade (slurry) or counterrotating roller (dry powder) is used. If per layer more than one type ofmaterial is applied, this is applied locally, preferably by means of oneor more computer-controlled nozzles capable of applying one or severalmaterials. The materials can differ from each other in color or inproperties. To be considered here are, for instance, (di)electric orpiezoelectric properties. According to this embodiment, the material ispreferably applied in the form of a slurry.

According to the invention, different kinds of materials, in particularboth ceramic materials and metals, can be used. To be able to properlyapply the material to the binder, the material is preferably in powderform. Depending on the size of the powder particles, the powder will beapplied in dry form or in dispersed form (slurry). A finer powder leadsto a greater accuracy in achieving the desired shape of the dentalelement. Preferably, the powder has an average particle size (diameter)between 1 nm and 50 μl, more preferably smaller than 50 nm, still morepreferably between 10 nm and 25 nm. The advantage of this is thatsintering can be carried out in a short time and at a relatively lowtemperature. It has been found that the particle size referred to has apositive effect on the shape and sinterability of the dental element tobe formed.

The powder can be made of any material that is conventionally used forforming dental elements. For this purpose, in particular metals andceramic materials and combinations thereof are suitable.

When a ceramic material is used for forming the dental element, this ispreferably selected from the group of SiO₂, Al₂O₃, K₂O, Na₂O, CaO, Ba₂O,CrO₂, TiO₂, BaO, CeO₂, La₂ ^(O) ₃, MgO, ZnO, Li₂O and combinationsthereof. Optionally, ceramic compositions can further contain F or P₂O₅.Particularly suitable ceramic materials are the commercially availablecompositions. VITADUR®, IPS EMPRESS®, DICOR®, IPS EMPRESS II®,CERESTONE®, CEREPHARAL®, and IN-CERAM®.

When a metal is used for forming the dental element, this is preferablyselected from the group of alloys of gold, platinum, palladium, nickel,chromium, iron, aluminum, molybdenum, beryllium, copper, magnesiumcobalt and tin. Optionally, such an alloy can contain silicon. For adescription of suitable alloys, reference is made to J. P. Moffa,Alternatives to Gold Alloys in Dentistry, DHEW Publication N. (NIH),77–1227.

If desired, a lubricant can be added to the powder to facilitateapplying the powder in layers. Examples of suitable lubricants arestearic acid or derived stearates, such as zinc or calcium stearate. Alubricant is preferably used in an amount of 1–2% by weight, based onthe weight of the powder.

As mentioned, preferably, in alternation a layer of binder is appliedand a layer of powder is applied thereto. The thickness of the layers ofpowder is preferably between 0.01 and 0.3 mm, more preferably between 20and 100 μm, which is beneficial to the surface quality in the case ofslight differences in height contour of the layers. The amount of binderper unit area of powder is fairly critical, but can simply be adjustedby a skilled person to the nature of the binder and powder used.Normally, the amount of binder will be between 0.005 and 0.3 grams persquare centimeter of powder. Thus, layer by layer the desired dentalelement is built up.

When the last layer has been applied, excess powder which has not beenbound is removed. This can be done by taking out the entire powder bed,turning it upside down and shaking gently. Residues can be removed byblowing, for instance with compressed ir. Thereafter the powderparticles can be bonded together by sintering. Preferably, prior tosintering, a debinding step is carried out, i.e., a treatment to removethe binder. Debinding can be carried out by means of heat or a suitablesolvent, such as hexane. Because most binders have a relatively complexcomposition, debinding preferably takes place by heating using atemperature path (for instance from 20 to 500° C.). Such a heatingscheme can be simply coupled to a sintering step.

The duration and temperature at which sintering takes place will dependon the nature of the binder used and the powder. Normally, the durationof sintering will be between 10 minutes and 3 hours, while thetemperature will typically be between 400 and 800° C. By sintering insuch a way that only necks are formed, shrinkage due to the sinteringstep is minimal/negligible. Optionally, such shrinkage can becompensated by scaling the CAD model.

After sintering, the product obtained is preferably infiltrated, wherebya second phase is introduced into the product. As a result, the porosityof the product is considerably reduced. Densities in excess of 99% arefeasible. The infiltration can be ed out, for instance, in an oven,whereby the infiltration material is laid against the dental element.The infiltration material melts at a lower temperature than the materialof the dental element. Through capillary action, the liquid infiltrationmaterial is infused (adsorbed). This step lasts a relatively short timeand gives the dental element the desired properties. A suitable materialor this is, for instance, glass-ceramic or a polymer. Preferably, amaterial is used which has been approved for use in dental elements, asdescribed in the standard ADA no. 15 ANSY MD156.15-1962, which is to beunderstood to be inserted herein.

In particular cases, it has been found to be advantageous to subject thedental element to a thermal/chemical post-treatment, so that all optimummaterial (micro)structure is achieved. Thus, preferably, the dentalelement is briefly heated to a temperature between 60 and 150° C., morepreferably between 80 and 130° C.

Instead thereof, or supplemental thereto, preferably a thermalcompaction is accomplished. To that end, the dental element is heated toa temperature of at least 250° C., preferably at least 400° C. and morepreferably at least 500° C. This treatment contributes to the dentalelement obtaining particularly favorable properties.

When by one of the procedures described above the dental element hasbeen formed, it may happen that it still needs to be additionally shapedto some extent. As has already been indicated, it is an advantage of theinvention that it enables work to be done very accurately. Additionalshaping will therefore be less laborious than in the techniques usedheretofore. Ways in which additional shaping can be carried out includeinter alia grinding, filing, polishing, sanding, blasting or treatmentwith a ball bed, depending on the selected material of the dentalelement.

The invention will presently be elucidated in and by the followingexamples.

Example 1

Two binders were prepared, having the following compositions:

A: polyvinyl acetate (Optapix PA 4 G) 2 wt. % alcohol content 36 wt. %(ethanol) glycol 2 wt. % water balance B: polyvinyl acetate (Optapix PA4 G) 2 wt. % alcohol content 34 wt. % (ethanol) glycol 1 wt. % waterbalance.

The compositions were prepared by manually adding the ingredients andstirring. Dissolving the polyvinyl acetate took 6 to 10 hours. Throughthe alcohol content, the surface tension could be set (a low surfacetension proved favorable).

Example 2

With a bindjet printer (Z402 of the firm Z Corporation, Burlington Mass.USA) two cylinders were fabricated, using aluminum powder (type CT3000SG) in combination with, successively, binder A and binder B (seeExample 1). The properties of the powder are as follows:

TABLE 1 Chemical purity (% by weight) Al₂O₃ >=99.7 Na₂O 0.09 SiO₂ 0.02Fe₂O₃ 0.02 CaO 0.02 MgO 0.10 Physical properties of the powder: Specificsurface energy range BET: 5.5 to 7.5 m²/g Median particle size (MPS)d50: 0.5 to 0.7 μm Cilas 850 Particle size d90: 1.0 to 2.0 μm Cilas 850Ceramic properties of the powder: Green density: 2.22 g/cm³ Sintereddensity: 3.90 g/cm³ Shrinkage: 16.5%

The alumina powder is distributed homogeneously over the buildingplatform by means of a divider (kind of razor blade/snow shovel/doctorblade). Thereafter, the layer of loose powder applied is compacted witha coated roller (teflon roller with polyester top layer), so that asmooth and flat layer of loose powder is formed (like flattened castorsugar). Through this compaction step, the initial porosity is renderedsubstantially lower, which is beneficial to the so-called greenstrength. The layer thickness of this powder layer is adjustable and hasbeen set at 0.0625 mm (the size of this step determines the accuracy offollowing the product contours, and may be still smaller).

After the entire building surface has been provided with a new compactedpowder layer, binder is locally applied to the loose powder by means ofa binder jet printer (Z402 of the firm Z Corp., see also WO-A-97/26802).The location where the binder substance is to be printed has beendetermined beforehand by software. The binder penetrates so deeply intothe loose powder that the powder particles in the new layer are bound toeach other and that further the new layer is bonded to the precedingone.

With the cartridge and binder substance used, an optimum in binderamount has been found to be 10× printing per 100 g. The amount of binderat a given layer thickness is 0.0017 g/cm² per inkjet run. Accordingly,at 10× ink jetting this is 0.017 g/cm², which leads to a goodconsistency of the products (they can be handled).

By repeating the recoating and inkjet steps, eventually the entireproduct is built up in the green (=with binder) form.

The cylindrical products which have been produced had a diameter of 16.4mm and a height of 18 mm; the mass is 5.3 g. The experiments werecarried out in triplicate. The porosity of the alumina cylinders is 45%at a maximum (in the absence of compacting). Compacting leads to a lowerporosity (estimate 55–70%).

The intermediate products were subsequently subjected to debinding andsintering according to a specific temperature-time path, whereby heatingwas done at a rate of 120° C. per hour to a temperature of 1200° C. Thistemperature was maintained for 120 minutes, followed by cooling to roomtemperature, again at a rate of 120° C. per hour. The sintered productswere subsequently infiltrated with a glass ceramic to obtain theeventual strength and mechanical properties. The obtained propertiessatisfy the standard imposed on the functional dental elements.

1. A method for fabricating a functional dental element using athree-dimensional printing technique comprising: applying successivelayers of powder onto each other to form the dental element; bonding thelayers by means of a binder wherein each layer is bonded at desiredpositions to a preceding layer thereby allowing removal of excessnon-adhering material; sintering the dental element to form necksbetween the powder particles; and subjecting the sintered dental elementto infiltration by second phase.
 2. A method according to claim 1,wherein the sintering step is preceded by a debinding step.
 3. A methodaccording to claim 1, wherein the shape and dimensions of the dentalelement are measured in a patient using an optical scan technique.
 4. Amethod according to claim 3, wherein the optical scan technique is alaser technique.
 5. A method according to claim 4, wherein the lasertechnique yields data about shape and dimensions in electronic form. 6.A method according to claim 1, wherein a computer is used forcontrolling, on the basis of the data obtained upon measuring, a printhead which applies the binder to specific, desired positions.
 7. Amethod according to claim 1, wherein the binder is selected from thegroup consisting of colloidal silica, polyvinyl acetate (PVA), starchadhesives, acrylates, polyvinyl alcohol, polyethylene oxide (PEO),ethylenevinyl acetate (EVA) and derivatives thereof.
 8. A methodaccording to claim 1, wherein the powder is a ceramic material, a metal,or a combination of metals and ceramic materials.
 9. A method accordingto claim 8, wherein the ceramic material is selected from the groupconsisting of SiO₂, Al₂O₃, K₂O, Na₂O, CaO, Ba₂O, CrO₂, TiO₂, BaO, CeO₂,La₂O₃, MgO, ZnO, Li₂O and combinations thereof.
 10. A method accordingto claim 8, wherein the metal is selected from the group consisting ofalloys of gold, platinum, palladium, nickel, chromium, iron, aluminum,molybdenum, beryllium, copper, magnesium, cobalt and tin andcombinations thereof.
 11. A method according to claim 1, wherein thelayers are applied with a doctor blade.
 12. A method according to claim1, wherein the powder is applied in dispersed form.
 13. A methodaccording to claim 12, wherein in a layer, the powder comprises powdersof different materials.
 14. A method according to claim 13, wherein in alayer, the powder comprises powders of different colors.
 15. A methodaccording to claim 12, wherein at least one layer differs in compositionfrom the other layers.
 16. A method according to claim 13, wherein thepowder is locally applied with a computer-controlled nozzle.
 17. Amethod according to claim 13, wherein at least one of the powders has anaverage particle size less than 50 nm.
 18. A method according to claim1, wherein the dental element is sintered at a temperature of 400–800°C. for a period between 10 minutes and 3 hours.
 19. A method accordingto claim 1, wherein said infiltration is carried out with aglass-ceramic or a polymer material.
 20. A method according to claim 1,wherein the dental element is further shaped by grinding, filing,polishing, sanding, blasting or treatment with a ball bed.